The present invention relates generally to the field of nondestructive testing of components and in particular to a new and useful method and apparatus for nondestructively testing tubes, such as are used in industrial boilers, for surface defects using electromagnetic acoustic transducer technology.
Electromagnetic acoustic transducers (EMATs) are known for use in testing materials for defects. In known systems, a signal generator creates an acoustic wave which propagates through a test material and either the original signal or a reflection is received by a single sensor having a coil for converting acoustic wave energy to an electrical current. EMATs are typically used on planar surfaces of a test material to detect both surface and sub-surface defects in the test material.
Tubes present a challenge for testing due to their curved surfaces. Tubes used in industrial boilers present a further challenge, as the space around and access to the tubes is typically very limited. These tubes must be as free of defects as possible, and coated with materials to resist corrosion and breakdown in the harsh environment of an industrial boiler.
Testing techniques such as conventional ultrasonic, eddy current, magnetic particle and dye penetrant testing all have limitations which prevent them from being used to perform high speed testing of 100% of the surface area of a tube. For example, ultrasonic testing requires a couplant material used with the sensor on the tube surface to ensure accurate signal pickup. Eddy current testing is susceptible to material variations and other properties within the material which can mask defects, or result in incorrect readings. Magnetic particle and dye penetrant testing each require a large amount of chemicals to coat the tubes and a long period of time to interpret results as well.
EMATs, by comparison, do not require couplants or chemicals, and since they rely on detection of the amplitude of acoustic signals which are converted to electrical signals, the testing may be done rapidly and the results displayed contemporaneously with the testing.